This application is a national stage of PCT/DE 98/03298 filed Nov. 5, 1998 and based upon German national application 19749836.1 filed Nov. 11, 1997 under the International Convention.
The invention relates to a process for separating a gas component from a gas mixture by freezing out.
From the publications DE 35 18 283 C2 and DE 196 35 002 A1, a process is known in which a gas to be continuously separated is evacuated from a cold trap. In this manner high degrees of purity are obtained.
A process of this type is also known from the publication DE 34 22 417 A1 for separating a gas component from a gas mixture by freezing out. A cold trap with a freezing stretch is provided. In the cold trap, the gas is subjected over a long residence time and on a very large cooling area to intensive heat exchange and mass exchange with the cooling or condensing surfaces. The cooling surfaces are generally formed as sheets running transversely to the gas flow direction.
A high degree of supersaturation of the gas within the cooling trap can thus be reliably avoided. The cold trap has no constrictions tending toward plugging up. The temperature is so controlled and monitored over the freezing stretch that at the gas input of the latter, the saturation temperature of the component to be frozen out prevails and at the outlet the saturation temperature corresponding to its desired residual concentration is maintained. Between the two, efforts are made to provide a substantially linear temperature profile. For that purpose, temperature sensors on the cooling sheets and a stepwise control of the stretch traversed by the coolant flow and additional heating with intervening heat damming stages are provided.
The known process requires a relatively high cost apparatus.
It is the object of the invention to provide a simple process of the type described in which the component to be separated can be obtained completely and with high purity. It is also an object of the invention to provide a process which can be carried out in a quasi continuous manner.
The objects are achieved with a process for separating out a gas component from a gas mixture by freezing out with the features:
a) the gas component to be separated has by comparison to the remaining gas components in the gas mixture, the smallest vapor pressure,
b) an operating temperature is selected at which the gas component to be separated is frozen out,
c) the gas mixture is subjected to the operating temperature,
d) the partial pressures of the remaining gas components at the prevalent working temperature are held smaller than their vapor pressures at the operating temperature, and
e) after the freezing out of the component to be separate, the solid left behind is separated from the remaining gases.
The remaining gases are separated by pumping them off from the solid left behind. The frozen gas separated from the remaining gases is then heated. An apparatus for carrying out the process can have
a) at least two cooling vessels which are cooled to the operating temperature by coolant are provided,
b) each of the two cooling vessels has a closeable inlet and outlet,
c) a pump is connected with each of both outlets,
d) means are provided which alternately feed the gas mixture to one and the other cooling vessel so that in the respective vessel a predetermined partial pressure is established.
The apparatus can have cooling sheets of increased surface area in the vessels.